The invention pertains to a photovoltaic device which comprises at least one active layer and a cover plate that contains on at least one side an array of optical structures and which is in optical contact with the light receiving surface of the active layer(s) in order to reduce the reflection losses of said surface. Said plate or sheet may also be used in combination with luminescent molecules, which are inside or in contact with said plate, to improve the spectral response of the photovoltaic device.
Photovoltaic devices are commonly used to convert light energy into electrical energy. These devices contain an active layer which consists of a light absorbing material which generates charge carriers upon light exposure. An active layer which is currently common in photovoltaic devices is silicon. However, a variety of materials can be encountered like for example gallium arsenide (GaAs), cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS). The charges, which are generated in the active layer, are separated to conductive contacts that will transmit electricity. Due to the thin and brittle nature of the active layer it is usually protected from external influences by a transparent cover plate e.g. made of glass. It is known from the art that both the active layer and the cover plate reflect a part of the light incident to the photovoltaic device. Especially the high refractive index of the active layer causes large reflection losses which can—in the case of silicon—be up to 22% of the incident light. Since the reflected light can not be converted into electrical energy these reflection losses cause a large reduction in the efficiency of a photovoltaic device.
Another effect that reduces the efficiency of a photovoltaic device is the low quantum efficiency of the active layer for usually short wavelengths, like for example ultra violet (UV) or blue light. This low response is caused by the band-gap of the material. The band gap refers to the energy difference between the top of the valence band and the bottom of the conduction band, where electrons are able to jump from one band to another. Due to the band-gap, the active layer has an optimal wavelength around which light energy is most efficiently converted into electrical energy. Light with a wavelength that is higher or lower than the optimum wavelength is less efficiently converted into electrical energy. A second effect that can reduce the spectral response of a photovoltaic device in the short wavelength range is the absorption of light by the cover plate. Although the cover plate is usually transparent to visible light it often absorbs in the UV range. As a result this light cannot reach the active layer of the photovoltaic device and cannot be converted into electrical energy.
In order to reduce these reflection losses, an anti reflection coating can be applied on top of the light absorbing material or so called active layer. An anti reflection coating consists of a single quarter-wave layer of a transparent material with a refractive index that is between the refractive index of the active layer and the cover plate. Although this theoretically gives zero reflectance at the center wavelength and decreased reflectance for wavelengths in a broad band around the center, the processing and material costs of these layers are relatively high. Also the processing techniques to create the coatings (e.g. chemical vapor deposition) are comprehensive and time consuming. In addition, the anti-reflection coating only works on the surface to which it is applied. It is therefore not possible to reduce both the reflection of the active layer and the cover plate by using one single anti reflection coating on either of these surfaces.
Another method to reduce the reflection losses is to structure the surface of the active layer. This can be done by either direct structuring of the material itself or by surface structuring of the substrate on which said material is deposited. By structuring the active layer, with common pyramid or V-shaped structures, a reduction in the reflection losses at active layer is obtained by multiple reflection at the surface offering the light a greater opportunity to enter the panel. This effect reduces the reflection losses at the surface of the active layer and is therefore often referred to as an anti-reflection effect. Secondly, the structures may in some cases partially trap the light which is not absorbed by the active layer and reflected by surface of the substrate. As a result the chance of light absorption by the active layer is increased. Although structuring of the active layer can significantly improve the efficiency of a photovoltaic cell, production methods are very complicated and extremely expensive. Often processes like wet chemical etching, mechanical etching or reactive ion etching are used to realize the desired effect. Also the structuring of the active layer does not reduce the reflection losses of the cover plate.
It is known from the art that the same concept as described in the previous paragraph can be used to improve the light transmission of a glass plat; i.e., the cover plate. Here, V-shaped (G. A. Landis, 21st IEEE photovoltaic specialist conference, 1304-1307 (1990)) or pyramidal structures as disclosed in WO 03/046617 are applied to a glass plate to reduce the reflection losses of said plate and hence increase its transmission. The structures can be applied to the glass plate via for example casting or pressing. However, when using the plate as a cover plate of a photovoltaic device, the maximum efficiency of said device can only be increased by 6%, which is a reduction of approximately 30% of the reflection losses, according to a model study (U. Blieske et al., 3rd World Conference on Photovoltaic Energy Conversion, 188-191 (2003)). In practice the results are even less and only 3% can be obtained. Although the structures reduce some of the reflection losses of the active layer, it reduces predominantly the reflection losses of the cover plate. Hence the total reduction in reflection losses, and increase in efficiency of the photovoltaic device, is low.